A surface acoustic wave filter (hereinafter, referred to as SAW filter) is used for a mobile phone and the like to select nothing but a required frequency band. In the SAW filter, it is required to widen the pass bandwidth, to increase suppression capability and to reduce insertion loss. With respect to such requirements, a structure of a ladder-type SAW filter in which a normal bandwidth is widened, insertion loss is reduced and suppression capability other than the normal bandwidth is increased is shown in JP-A-5-183380. According to this, a first surface acoustic wave resonator with a pair of terminals having a prescribed resonant frequency is arranged in a parallel arm, and a second surface acoustic wave resonator with a pair of terminals having a resonant frequency which almost corresponds to an anti-resonant frequency of the first resonator is arranged in a series arm. Further, a structure that an inductance is added in series to the first surface acoustic wave resonator is also disclosed.
Not only the above structure but also a various kinds of structures for realizing a better filter characteristic are proposed and put into practical use.
SAW filter 200 having a structure shown in FIG. 14 is generally used in many cases. SAW filter 200 shown in FIG. 14 includes three surface acoustic wave resonators in series arms 204, 212, 220 and two surface acoustic wave resonators in parallel arms 228, 236 formed on piezoelectric substrate 202, and a prescribed filter characteristic is obtained by connecting these resonators. Specifically, surface acoustic wave resonators in series arms 204, 212, 220 respectively include interdigital transducer electrodes (hereinafter, referred to as IDT electrodes) 206, 214, 222 and reflector electrodes 208, 210, 216, 218, 224, 226 disposed at both sides. Surface acoustic wave resonators in parallel arms 228, 236 also include IDT electrodes 230, 238 and reflector electrodes 232, 234, 240, 242 disposed at both sides respectively. Three surface acoustic wave resonators in series arms 204, 212, 220 are connected in series through first connecting wirings 242, 244. Surface acoustic wave resonators in parallel arm 228, 236 are connected to second connecting wirings 246, 248 connecting to first connecting wirings 242, 244, and the other sides are connected to ground 250. Further, surface acoustic wave resonators 204, 220 disposed at outer sides respectively in three surface acoustic wave resonators 204, 212, 220 in series arms are connected to input terminal 252 and output terminal 254.
According to the structure, SAW filter 200 having the prescribed characteristic can be realized. However, in order to increase out-of-band attenuation in the structure, it is required to increase the number of surface acoustic resonators. For the purpose, the chip size has to be increased.
The invention solves the conventional problems, and an object thereof is to provide a SAW filter that can be miniaturized while having a high attenuation characteristic.